The present invention relates to the field of semiconductor processing, and, more particularly, to an apparatus and method for generating a pattern on a photomask.
In the photolithographic fabrication of integrated circuits, a film sensitive to optical beams is exposed in predetermined patterns to define circuit features. Optical beams are passed through masks which contain the patterns, thereby selectively exposing the photosensitive film on a semiconductor body. In other instances, the photosensitive film is on a mask substrate and the film is exposed as a step in the making of the photomask.
A system for making a photomask is disclosed in U.S. Pat. No. 5,386,221 to Allen et al. A laser beam is used to expose a photosensitive film for defining a pattern on a photomask. A beam splitter splits the laser beam into 32 individual optical beams to create what is commonly referred to as a brush. The beam splitter is also referred to as a brush module. Increased print speed is accomplished through the use of a wider brush. A scanner scans the 32 optical beams across the photosensitive material. The optical beams are modulated by an acoustic-optical modulator (AOM) to define the pattern to be generated on the photomask.
The beam splitter disclosed in Allen et al. produces 32 optical beams which are evenly spaced apart except for the two central beams which have a different spacing referred to as a diastemal split. The diastemal split plays a role in generating the proper error averaging during successive passes of the optical beams in the printing process.
To produce the 32 optical beams, 5 spaced apart high precision mirrors are used. Initially, a single optical beam enters a first splitter of thickness t which splits the optical beam into 2 optical beams. The splitter has a first surface which reflects 50% of the optical beam, a transparent material, and a second surface which reflects the remaining 50% of the optical beam. The distance between the two optical beams is determined by the distance between the first and second surfaces, i.e., the thickness of the transparent material.
The two optical beams are reflected from a second splitter of thickness 2t. The second splitter creates 4 beams. The 4 beams are reflected from a splitter of thickness 4t which creates 8 beams. These 8 beams are reflected from another splitter of thickness 8t which creates 16 beams. The remaining 16 beams are reflected from a fifth splitter having a thickness of 16.5t to create the desired 32 beams. The fifth splitter is thicker than twice the thickness of the fourth splitter. This creates the disatemal split between the two centermost optical beams. A disadvantage of this approach is that these mirrors are expensive to produce and require precise alignment with respect to one another.
In view of the foregoing background, it is therefore an object of the present invention to generate a plurality of spaced apart optical beams with a different or enlarged beam spacing between two of the optical beams without using a series of high precision mirrors.
This and other objects, features and advantages in accordance with the present invention are provided by an apparatus for generating a pattern on a photomask comprising a beam generator generating a row of spaced apart optical beams, a blocking element downstream from the beam generator in a path of at least one of the optical beams, and a refracting element downstream from the blocking element for aligning optical beams therefrom closer together and while leaving an enlarged beam spacing downstream from the at least one blocked optical beam The apparatus preferably further comprises a modulator downstream from the refracting element for defining the pattern to be generated on the photomask, and a scanner for causing the optical beams to scan across the photomask.
The beam generator preferably comprises a laser generating an optical beam, a diffractive optical device downstream from the laser and generating a plurality of optical beams, and a collimator downstream from the diffractive optical device for collimating the plurality of optical beams into the row of spaced apart optical beams. In one embodiment, the optical beams preferably have a substantially equal beam spacing. The blocking element preferably blocks a center optical beam, and the enlarged beam spacing downstream from the blocking element is preferably greater than about one times and less than about two times the substantially equal beam spacing.,
The optical beams from the beam generator may be 33 in number, and the blocking element may block the center one of the 33 optical beams. The apparatus according to the present invention advantageously generates the row of spaced apart optical beams with the enlarged beam spacing between the two remaining central beams without using a series of high precision mirrors that are expensive to produce and require precise alignment with respect to one another.
The refracting element preferably comprises silica. In one embodiment, the refracting element preferably comprises a pair of angled refracting portions defining an apex aligned with the blocking element. The beam generator preferably generates the optical beams so that each has a wavelength within a range of about 190 to 400 nm.
Another aspect of the invention relates to a method for generating a pattern on a photomask. The method preferably comprises generating a plurality of optical beams, blocking at least one of the optical beams, and aligning the unblocked optical beams closer together and defining an enlarged beam spacing downstream from the at least one blocked optical beam. The method preferably further comprises modulating the aligned optical beams for defining the pattern to be generated on the photomask, and scanning the modulated optical beams across the photomask.